Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving surface. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink or an ink emulsion. Other inkjet printers receive ink in a solid form and then melt the solid ink to generate liquid ink for ejection onto the image receiving surface. In these solid ink printers, also known as phase change inkjet printers, the solid ink can be in the form of pellets, ink sticks, granules, pastilles, or other shapes. These solid forms are denoted by the term “solid ink sticks” in this document. The solid ink sticks are typically placed in an ink loader and delivered through a feed chute or channel to a melting device, which melts the solid ink. The melted ink is then collected in a reservoir and supplied to one or more printheads through a conduit or the like. Other inkjet printers use gel ink. Gel ink is provided in gelatinous form, which is heated to a predetermined temperature to alter the viscosity of the ink so the ink is suitable for ejection by a printhead. Once the melted solid ink or the gel ink is ejected onto the image receiving member, the ink returns to a solid, but malleable form, in the case of melted solid ink, and to a gelatinous state, in the case of gel ink.
A typical inkjet printer uses one or more printheads with each printhead containing an array of individual nozzles through which drops of ink are ejected by inkjets across an open gap to an image receiving surface to form an ink image during printing. The image receiving surface can be the surface of a continuous web of recording media, a series of media sheets, or the surface of an image receiving member, which can be a rotating print drum or endless belt. In an inkjet printhead, individual piezoelectric, thermal, or acoustic actuators generate mechanical forces that expel ink through apertures, usually called nozzles, which are arranged in a faceplate of the printhead. The actuators expel an ink drop in response to an electrical signal, sometimes called a firing signal. The amplitude or duration of the firing signals affects the amount of ink ejected in an ink drop. The firing signal is generated by a printhead controller with reference to image data.
A print engine in an inkjet printer is comprised of a processor that executes instructions stored in a memory operatively connected to the processor to process image data also stored in a memory operatively connected to the processor to identify the inkjets in the printheads of the printer that are operated to eject a pattern of ink drops at particular locations on the image receiving surface to form an ink image corresponding to the image data. The locations where the ink drops landed are sometimes called “ink drop locations,” “ink drop positions,” or “pixels.” Thus, a printing operation can be viewed as the placement of ink drops on an image receiving surface with reference to electronic image data.
Phase change inkjet printers form images using either a direct or an offset print process. In a direct print process, melted ink is jetted directly onto recording media to form images. In an offset print process, also referred to as an indirect print process, melted ink is jetted onto a surface of a rotating member such as the surface of a rotating drum, belt, or band. Recording media are moved proximate the surface of the rotating member in synchronization with the ink images formed on the surface. The recording media are then pressed against the surface of the rotating member as the media passes through a nip formed between the rotating member and a transfix roller. The ink images are transferred and affixed to the recording media by the pressure in the nip. This process of transferring an image to the media is known as a “transfix” process.
A known system for ejecting ink to form images on a moving web of media material is shown in FIG. 4. The system 10 includes a web unwinding unit 14, a printing apparatus 18, and a cutting station 22. In brief, the web unwinding unit 14 includes an actuator, such as an electrical motor, that rotates a roll of media material in a direction that removes a web 26 of media material from the unwinding unit 14. The web 26 is fed through the printing apparatus 18 along a path, which extends to the cutting station 22. The printer, referred to as a printing apparatus 18, treats the web 26 to remove debris and loose particulate matter from the web surface, ejects ink using data and signals generated by one or more print engines onto the moving web to form ink images. A print engine can include one or more marking stations having one or more printheads. Once the printed image has been applied to the web, the printer fixes the printed image to the web. The marking stations can be configured to eject different colored inks onto the web 26 to form a composite colored image. In one system 10, the marking stations eject cyan, magenta, yellow, and black ink for forming composite colored images. The web 26 is then pulled into the cutting station 22, which cuts the web into sheets for further processing.
The printing apparatus 18 is configured with one or more processors, programmed instructions, and electronic components to implement a registration control method that controls the timing of the ink ejections onto the web 26 as the web passes the marking stations. One known registration control method that may be used to operate the marking stations in the printing apparatus 18 is the single reflex method. In the single reflex method, the rotation of a single roller at or near a marking station is monitored by an encoder. The encoder may be a mechanical or electronic device that measures the angular velocity of the roller and generates a signal corresponding to the angular velocity of the roller. The angular velocity signal is processed by a controller executing programmed instructions for implementing the single reflex method to calculate the linear velocity of the web. The controller may adjust the linear web velocity calculation by using tension measurement signals generated by one or more loadcells that measure the tension on the web 26 near the roller. The controller implementing the single reflex method is configured with input/output circuitry, memory, programmed instructions, and other electronic components to calculate the linear web velocity and to generate the firing signals for the printheads in the marking stations.
Another known registration control method that may be used to operate the marking stations in the printing apparatus 18 is the double reflex method. In the double reflex method, two rollers are monitored by an encoder. One roller lies on the web path before the marking stations and the other roller lies on the web path following the marking stations. The angular velocity signals generated by the encoders for the two rollers are processed by a controller executing programmed instructions for implementing the double reflex method to calculate the linear velocity of the web 26 at each roller and then to interpolate the linear velocity of the web at each of the marking stations. These additional calculations enable better timing of the firing signals for the printheads in the marking stations and, consequently, improved registration of the images printed by the marking stations in the printing apparatus 18.
To address demand for printing systems that use a large number of colored inks, some printing systems include more than one printing apparatus. For instance, in a tandem printing system, a tandem printing system can include two of the printing apparatus 18, such as the one shown in FIG. 4, arranged in a tandem configuration. The tandem configuration enables the marking stations in each of the two printing apparatus 18 to use different colored inks. Additionally, a web inverter may be positioned between the two printing apparatus 18 to enable the web to be turned over so the reverse surface of the web may be printed by the second printing system. The tandem printing system configuration enables the entire width of the reverse side of the web to be printed.
One issue encountered in printing systems having a first printing apparatus and a second printing apparatus arranged serially in tandem is the need to synchronize the registration of images being printed by the first and the second printing apparatus. If the two serially connected printing apparatus 18 form images on the same side of the web, then slight differences in the printed images can adversely impact image quality. Even when the two printing apparatus 18 form images on different sides of the web, registration is still important because the duplex printed web is cut into individual, double-sided printed pages. If the registration of images is not accurately controlled, an image on one side of the web may creep over the length of a print job into the cutting zone between images.
In a tandem web printing system, two print engines with one engine being located in each printer, should print images on the web at substantially the same speed. Each of the print engines includes a print driver, typically a drum, coupled to a motor to move the web past respective printheads. By coordinating the speed of the first print engine with the speed of the second print engine, the amount of web located between the first and the second printer can be controlled to prevent the web from tearing during printing or falling to a floor or another location situated between printers. In some tandem printing systems a web buffer, also known as a loop box, provides for web transport between the first printer to the second printer. The web buffer accommodates a certain amount of slack, or sag, that can be present between print engines should the print engines be running at different speeds or should certain system components be deficient to meet design constraints. The web buffer includes a sensor to detect the depth, or amount of sag, of a loop of web material passing between two rollers. If the detected depth falls outside a predetermined maximum depth, print engine speed can be adjusted to maintain acceptable tandem printing system web motion. Since the performance of printing process registration can be directly related to a change in speed over time between the print engines and respective print drivers, a smaller change in speed over time provides for a better registration performance. On the other hand, a smaller change in speed over time can necessitate a larger loop control box since the amount of slack or the depth of the loop in the loop control box can become excessive. Consequently, improvements to the registration of the images printed by the two printing apparatus on a single web would be desirable. Thus, accurate control of one print engine with respect to another print engine in a tandem printing system is desirable.